1. Field of the Invention
The present invention relates to a method and an apparatus for producing reduced iron by mixing a powder of an iron material and a powder of a reducing agent to form a mixed powder, agglomerating the mixed powder to form compacts like briquettes or pellets, and reducing the compacts in a high temperature atmosphere.
2. Description of the Related Art
FIG. 8 outlines a production process by a conventional apparatus for producing reduced iron.
In a conventional apparatus for producing reduced iron, as shown in FIG. 8, an iron ore powder, a coal powder, and a binder are mixed in a mixer (not shown) . The resulting mixed powder is agglomerated by a pelletizer or a briquetter 001 to form green compacts (raw compacts). Then, the green compacts are charged into a dryer 002, where they are dried with an off-gas from a reducing furnace (a rotary hearth furnace, RHF) 004 to be described later on. The so dried green compacts are fed to the RHF 004 by a compact feeder 003. The interior of the RHF 004 is heated by burners 005, and thereby maintained in a high temperature atmosphere. The off-gas inside the RHF 004 is discharged from an off-gas duct 006.
The green compacts are heated with the radiant heat of a high temperature gas while they are moving in the RHF 004. Iron oxide in the iron ore is reduced with coal to form reduced iron in compact form. The reduced compacts are discharged by a compact discharger 007, and accommodated into a reservoir 008. The off-gas discharged through the off-gas duct 006 is cooled by a primary cooler 009, and then sent to a heat exchanger 010, where the cooled off-gas is heat exchanged. Air heated upon heat exchange is sent to the RHF 004, and fed into the furnace together with fuel. On the other hand, the off-gas is cooled again by a secondary cooler 011, and a part of the off-gas is sent to the dryer 002 as drying air for the green compacts, as stated earlier. The off-gas discharged from the dryer 002 is cleaned by a dust collector 012, and then released into the atmosphere.
The reservoir 008 accommodating the reduced compacts is passed on to a subsequent step. That is, the reduced compacts in the reservoir 008 are supplied to a raw material tank (hopper) 013, and charged into a melting furnace 015 via a chute feeder 014 for melting.
The RHF 004 of the foregoing apparatus for producing reduced iron requires that the residence time of the green compacts in the high temperature atmosphere be minimized for increased productivity. Thus, the interior of the RHF 004 needs to be heated to a high temperature of 1,200 to 1,300xc2x0 C. The reduced compacts discharged from the compact discharger 007 are accommodated at a high temperature directly into the reservoir 008. In the reservoir 008, the reduced compacts stick to each other under their own weight. When the reduced compacts are charged from the reservoir 008 into the melting furnace 015 through the chute feeder 014, large lumps of the reduced compacts stuck together may clog the chute feeder 014.
Hence, it has been common practice to dispose a rotary drum type cooler immediately below the compact discharger 007 of the RHF 004, cool the hot temperature reduced compacts to ordinary temperature in this cooler, and then accommodate them into the reservoir 008. This necessitates equipment cost for the cooler, and requires a cooling time for cooling the reduced compacts to ordinary temperature. Thus, the productivity is decreased, and the forced cooling of the high temperature reduced compacts wastes the heat that the compacts per se retain.
Moreover, the high temperature atmosphere is unstable during an initial period of operation of the RHF 004. In this case, the reduced compacts are reoxidized, and heat generation during this reoxidation results in partial melting. As a result, the reduced compacts stick to each other, forming large lumps. In this case as well, when the reduced compacts are charged from the reservoir 008 into the melting furnace 015 through the chute feeder 014, large lumps of them may clog the chute feeder 014, as stated previously.
The present invention has been accomplished to solve the above-mentioned problems. It is an object of this invention to provide a method and an apparatus for producing reduced iron, which eliminate operating defects in a subsequent step due to large lumps of reduced compacts, and prevent a decrease in the efficiency of production.
A method for producing reduced iron according to the present invention, as a means of attaining the above-mentioned object, is a method for producing reduced iron by agglomerating a mixed powder of an iron material and a reducing agent to form compacts like briquettes or pellets, and reducing the compacts in a high temperature atmosphere, wherein when a temperature of reduced compacts is 900xc2x0 C. or higher, and an oxide content in the reduced compacts is 11% or more.
Thus, mutual sticking of the reduced compacts can be suppressed to eliminate operating defects in a subsequent step due to large lumps of the reduced compacts, and prevent a decrease in the efficiency of production.
In the method for producing reduced iron according to the invention, basicity of the reduced compacts may be 0.5 or more. Since the basicity of the reduced compacts is 0.5 or more, mutual sticking of the reduced compacts can be suppressed reliably.
In the method for producing reduced iron according to the invention, the compacts maybe formed, with the amount of limestone mixed with the mixed powder of the iron material and the reducing agent being adjusted so that the oxide content in the reduced compacts will be 11% or higher. Thus, the adjustment can be made easily and highly accurately so that the oxide content in the reduced compacts will be appropriate.
An apparatus for producing reduced iron according to the invention is an apparatus for producing reduced iron by agglomerating a mixed powder of an iron material and a reducing agent to form compacts like briquettes or pellets, and reducing the compacts in a high temperature atmosphere, the apparatus further including grinding means for grinding reduced compacts.
Since large lumps of reduced compacts stuck together are ground by the grinding means, operating defects in a subsequent step due to large lumps of the reduced compacts can be eliminated, and a decrease in the efficiency of production can be prevented.
In the apparatus for producing reduced iron according to the invention, the grinding means may be disposed in a discharge port of an RHF, and the reduced compacts ground by the grinding means may be accommodated and stored in a reservoir. Thus, the reduced compacts in a high temperature state can be easily ground, and can be stored without being oxidized again.
In the apparatus for producing reduced iron according to the invention, sifting means may be provided for sifting the reduced compacts according to size of the compact, and large lumps of the reduced compacts sifted out by the sifting means may be ground by the grinding means. Since only large lumps of the reduced compacts are ground by the grinding means, the amount of operation of the grinding means can be decreased, and the efficiency of processing can be increased.
In the apparatus for producing reduced iron according to the invention, the sifting means may be composed of a plurality of sieves, and a vibrator for vibrating the sieves, each of the sieves being composed of a plurality of rods supported at predetermined intervals and in an inclined state on an upper portion of a body. Thus, the structure of the sifting means can be simplified and made lightweight.
In the apparatus for producing reduced iron according to the invention, reduced compacts discharged from an RHF may be sifted by the sifting means, then large lumps of the reduced compacts may be ground by the grinding means, and small lumps of the reduced compacts sifted out by the sifting means and small lumps of the reduced compacts formed by grinding by the grinding means may be accommodated and stored in a reservoir. Thus, the reduced compacts in a high temperature state can be easily ground, and can be stored without being oxidized again.
In the apparatus for producing reduced iron according to the invention, temporary storage means may be provided for temporarily storing the large lumps of the reduced compacts sifted out by the sifting means, and after a predetermined amount or a larger amount of the large lumps of the reduced compacts are stored in the temporary storage means, all of these large lumps may be ground by the grinding means. Thus, the large lumps of the reduced compacts stuck together in a transport reservoir are ground by the grinding means. Consequently, operating defects in a subsequent step due to large lumps of the reduced compacts can be prevented reliably.
In the apparatus for producing reduced iron according to the invention, a reservoir for accommodating and storing reduced compacts may be provided in a discharge port of an RHF, the grinding means may be disposed in a charge port for a raw material tank in a melting furnace, and immediately before the reduced compacts in the reservoir are charged into the raw material tank, large lumps of the reduced compacts may be ground by the grinding means. Thus, after the reduced compacts become cold, the large lumps are ground. Consequently, the reduced compacts can be prevented from resticking to each other.
In the apparatus for producing reduced iron according to the invention, sifting means for sifting the reduced compacts according to the size of the compact may be provided in the charge port for the raw material tank in the melting furnace, and large lumps of the reduced compacts sifted out by the sifting means may be ground by the grinding means and then charged into the raw material tank. Thus, after the reduced compacts become cold, only large lumps of the reduced compacts are ground by the grinding means. Consequently, resticking of the reduced compacts to each other can be prevented, the amount of operation of the grinding means can be decreased, and the efficiency of processing can be increased.